The antenna is a converting device designed for sending or receiving the electromagnetic wave, which can convert the electromagnetic wave into the current, and vice versa. The voltage standing wave ratio (VSWR) of the antenna is commonly used to estimate the matching status between the impedance value of the transmission wire and that of the antenna. It is well-known by the skilled person that VSWR=Vmax/Vmin=(1+|Γ|)/(1−|Γ|), wherein Vmax represents the maximum voltage value of the standing wave, Vmin represents the minimum voltage value of the standing wave, and Γ represents the reflection coefficient. It is also well-known by the skilled person that Γ=(Z−Z0)/(Z+Z0), wherein Z is the impedance value of the antenna, and Z0 is the impedance value of the transmission wire. Therefore, the impedance value of the antenna Z will affect the reflection coefficient, thereby indirectly affecting the VSWR, i.e. the matching status between the impedance value of the transmission wire and that of the antenna. Hence, the impedance value of the antenna needs to be considered when designing the antenna. When designing the antenna, the receiving or transmitting frequency of the antenna, the gain of the antenna, the radiation power of the antenna, the return loss of the antenna, the length and geometric figure of the antenna, and the matching between the impedance value of the transmission wire and the impedance value of the antenna also need to be considered.
Currently, the size of the wireless product tends to miniaturization. The antenna is an important element of the wireless product so that it also tends to miniaturization.
Please refer to FIG. 1, which shows a conventional dual-band loop-type antenna in the Taiwanese Patent No. I319643. The conventional dual-band loop-type antenna 1 includes a ground surface 11, an FR4 glass substrate 10, a radiation metal ring 12 and a radiation metal sheet 13. The ground surface 11 includes a ground point 111 and a short-circuit point 112. The radiation metal ring 12 has a feed terminal 121 and a short-circuit terminal 122, and there is a specific distance between the feed terminal 121 and the short-circuit terminal 122. The short-circuit terminal 122 is electrically connected to the short-circuit point 112 on the ground surface 11, and the specific distance between the feed terminal 121 and the short-circuit terminal 122 is less than 5 mm. The radiation metal sheet 13 has a terminal 131, and is surrounded by the radiation metal ring 12. The terminal 131 of the radiation metal sheet 13 is electrically connected to the vicinity of the short-circuit terminal 122 of the radiation metal ring 12, and the distance between the terminal 131 and the short-circuit terminal 122 is less than 10 mm.
The size of the ground surface 11 is 50*100 mm2, and the area surrounded by the radiation metal ring 12 is 50*15 mm2. Since the conventional dual-band loop-type antenna 1 occupies more space, it is not suitable for the small wireless product. Besides, the conventional dual-band loop-type antenna 1 has a small bandwidth, and is only suitable for the central frequency 900 MHz with a bandwidth of 250 MHz as well as the central frequency 1800 MHz with a bandwidth of 170 MHz. Moreover, the conventional dual-band loop-type antenna 1 uses the printed circuit board and the etching technology to be formed, together with the ground surface 1, on the FR4 glass substrate 10 with a thickness of 0.8 mm. This not only requires a more complicated process and a higher cost, but also reduces the radiation power of the antenna.
In order to overcome the drawbacks in the prior art, a loop-type antenna is provided. The particular design in the present invention not only solves the problems described above, but also is easy to be implemented. Thus, the present invention has the utility for the industry.